It is estimated that four to five million epileptics in he United States are treated with 5, 5-diphenylhydantoin (PHT, phenytoin). Of this population, approximately two to three million individuals ("responders") develop gingival overgrowth as an undesirable effect of the drug. Studies in human and model mongrel cat populations have suggested that PHT metabolites may be responsible for development of the lesion by selecting for a subpopulation of synthetically hyperactive gingival fibroblasts, which proliferate in susceptible individuals. A population of pediatric patients on PHT therapy will be classified as responder or non-responder individuals on the basis of oral examinations. Urinary metabolites from such individuals will be quantitated by gas chromatography and high-pressure liquid chromatography (HPLC) to determine the state of conjugation, stereochemical composition, and relative amounts of phenolic (p-HPPH, 5-(4-hydroxyphenyl)-5phenylhydantoin) and dihydrodiol (DHD, 5-(3, 4-dihydroxy-1, 5-cyclohexadien-1-yl)-5-phenylhydantoin), and other minor metabolites of PHT. These data will be used to determine if metabolic differences exist between the human responder and non-responder groups. Gingival fibroblast cultures from responder, non-responder, and p-HPPH and DHD to determine what cytotoxic or mitogenic effects such compounds may possess. The mode of PHT metabolism by in vitro gingival firbroblast cultures will be examined and compared by use of HPLC techniques. The stereochemistry of urinary p-HPPH and DHD in responder and non-responder mongrel cats will be compared. The ability of racemic p-Hpph and its enantiomers to induce gingival overgrowth in responder mongrel cats will also be evaluated. Such studies will allow evaluation of potentially toxic arene oxide and other metabolic pathways in responder and non responder individuals, the biological activity of such metabolic products, and the sites (liver and/or gingivae) of production of the active metabolites. With the added insights into the biochemical mechanism of the induction of gingival overgrowth, it may be possible to develop screening procedures to identify potentially susceptible individuals. New theories for the design of less-toxic antepileptic drugs may also be developed.